Hi,
Every one of your workers will need a cscs card of their own, if they are to work on a building site.
We had to put all our guys through it a few months ago. The test is pretty easy, spend an afternoon reading the revision book and you should have no problems.
The CSCS scheme has been backed by the uk's major house builders, so to win contracts for large new build cleaning work you will have to take the health and safety test and get a card.
Cheers,
Huw.